TEM is a widely used reference chemical mutagen, both as a positive control in routine testing and as the mutagen of choice in the developmental aspects of the mutagenesis programs. The goal of this project is to gain a better understanding of the mode of action of TEM through studies of the pharmacokinetics of this compound. Our results indicate a rapid turnover and elimination of TEM regardless of the placement of the label. However, we have found that with oral administration of TEM, the ethyleneimine portion of the molecule enters the blood in relatively small amounts as compared to i.p. injections.